The organosilanes most often utilized in surface modification are silicon chemicals which possess a single hydrolytically sensitive center that can react with inorganic substrates, such as glass, to form stable covalent oxane bonds. These organosilanes also possess an organic substitution which functions to alter the physical interaction of the modified surface with various substrates. The most widely used silanes modify inorganic substrates, such as fiberglass, in order to create a bond with an organic material, such as a polymer, to form reinforced composites. These silanes are often referred to as coupling agents (see, for example, B. Arkles, “Tailoring Surfaces with Silanes,” CHEMTECH 7, 766-778 (1977); E. Plueddemann, “Silane Coupling Agents,” Plenum, 2nd edition (1990)).
Dipodal silanes are silanes that are employed in surface modification and possess two silicon atoms, both of which are capable of bonding to inorganic surfaces through oxane bonds. The term dipodal derives from the Greek suggesting “two feet on the ground,” corresponding to the ability of this class of compounds to react with inorganic substrates at two different centers. A salient characteristic differentiating dipodal silanes from conventional silanes is their ability to form bonds that simultaneously exhibit greater hydrolytic stability and greater mechanical strength than the corresponding conventional monopodal silanes.
For the most part, dipodal silanes utilized both commercially and reported in the literature may be regarded as “bridged,” that is, each silane substitution is at the terminal end of an organic substitution. Examples of non-functional bridged dipodal silanes include 1,2-bis(trimethoxysilyl)ethane and 1,8-bis(triethoxysilyl)octane. Examples of functional bridged dipodal silanes include bis(trimethoxysilylpropyl)amine and bis(triethoxysilylpropyl)tetrasulfide. However, it has been observed that resistance to hydrolysis and the ability to interact with resins does not appear to be optimized when there are a large number of atoms between the silicon centers and when the functionality is part of the bridging group. An explanation for this behavior may be that a relatively dense siloxane network forms when the hydrolyzable groups are in close proximity to each other, and the ability of the functional group to extend away from the treated surface may be encumbered when it is part of a bridging group.
Relatively few examples of pendant dipodal silanes have been reported. One example is described in U.S. Pat. No. 7,235,683, in which double hydrosilylation of a terminal double bond creates a dipodal silane having the silicon atoms are separated by two carbon atoms. U.S. Pat. No. 7,265,236 provides other examples of dipodal silanes that may be regarded as pendant, but in these cases the two silicon atoms are significantly separated from each other. Accordingly, dipodal silanes in which the silicon centers are in close proximity to each other would be desirable, particularly for use in surface modification.